1. Field of the Invention
This invention relates generally to methods, systems and apparatus for managing digital communication systems. More specifically, this invention relates to adaptive control of various transmission parameters, including but not limited to maximum transmit power spectral density, maximum aggregate transmission power, transmission band preference, minimum and maximum receiver margin, frequency-dependent bit-loading and power controls and/or bit-loading restrictions in communication systems such as DSL systems.
2. Description of Related Art
Digital subscriber line (DSL) technologies provide potentially large bandwidth for digital communication over existing telephone subscriber lines (referred to as loops and/or the copper plant). Telephone subscriber lines can provide this bandwidth despite their original design for only voice-band analog communication. In particular, asymmetric DSL (ADSL) can adjust to the characteristics of the subscriber line by using a discrete multitone (DMT) line code that assigns a number of bits to each tone (or sub-carrier), which can be adjusted to channel conditions as determined during training and initialization of the modems (typically transceivers that function as both transmitters and receivers) at each end of the subscriber line. The adaptive assignment can be continued during live data transmission on channels or lines that vary with time using a process often referred to as “bit-swapping” that uses a secure relatively low-speed reverse channel to inform the transmitter of assignment changes.
Impulse noise, other noise and other sources of error can substantially impact the accuracy of data transmitted by ADSL and other communications systems. Various techniques have been developed for reducing, avoiding and/or repairing the damage done to data by such error during transmission. These error reduction/avoidance/repair techniques have performance costs for a communication system in which they are used. As is well known in the art, inadequate power transmission levels lead to errors because the transmission power is not high enough to overcome noise and other interference in a given channel. These errors lead to lost data and/or the need for re-transmission of data, sometimes multiple times. To prevent such errors, systems utilize extra transmission power that results in margins above a known or calculated signal-to-noise ratio (SNR) that assures compliance with an acceptable error rate.
Generally, DSL modem pairs determine power, margin and other operational characteristics of the pair during initialization, training, channel analysis and exchange phases before full operation (sometimes referred to as SHOWTIME). The process starts with a base power spectral density (PSD) value or mask. This may be a “flat” or constant value (that is, frequency independent) or may be a variable mask where the PSD value is frequency-specific or frequency-dependent. There is an initial PSD value in various DSLs (sometimes referred to as “NOMPSD”) and typically an upper limit for NOMPSD is defined by an applicable standard for a given country. Using this initial PSD value, the modems estimate line attenuation and line length (and perhaps other parameters and/or values).
Using the line attenuation and length estimates, one or both modems can define a power drop, or power cut-back (PCB) value, that reduces the initial PSD value. As noted below, different DSL standards set and adjust power (PSD and PCB, for example) according to different rules, if the standards are indeed observed and obeyed at all.
Using the once-adjusted PSD value (sometimes referred to as REFPSD=PSD−PCB), the modems compute the bit-loading (bi), gains (gi) and margin during the channel analysis phase. The gains gi are adjustments to individual bit-loaded tones' transmit power levels in the DMT scheme, providing a relatively uniform margin for transmission of data on the line. The gains can be adjusted during SHOWTIME to reflect changing line conditions, etc., but may be very limited as to the amount of adjustment and the way such gains adjustments can be performed.
Depending on the equipment manufacturer and the DSL standard being used, compliance with each standard's rules and guidelines and/or other appropriate operational limits varies from strict compliance by some parties (usually leading to very conservative or timid setting of DSL service rates in operation) to grave disregard for even basic operational guidelines and rules. In many cases, whether through intentional or inadvertent non-compliance, excessive power and/or margins are used in an attempt to avoid problems that can arise from too little of either or both.
Excessively high power transmission levels, however, lead to other problems. For example, one or more lines' use of excessive transmission power can generate strong crosstalk problems and interference in nearby lines. Crosstalk is unwanted interference and/or signal noise electromagnetically passed between lines that share the same or adjacent binders. In addition, use of transmission power above necessary levels also means that the communication system is operated more expensively, to the detriment of all users. Following is a brief summary of existing standards and practices for several modes of DSL service to which embodiments of the invention disclosed below may apply, including nuances that differentiate each particular standard from the more generic initialization, training, channel analysis and exchange procedure discussed above.
ADSL1-G.992.1 Standard (Also Referred to Herein as the “ADSL1 Standard” or “ADSL1”):
(1) Has a MAXSNRM maximum margin (or equivalent) limit setting that can be specified by operators, but the capability to observe and implement this limit varies with modem manufacturers and interpretations of the standard, with the result that it is often effectively ignored. Generally, an “operator” is a telecom or other service provider who operates the network and provides the service itself. Internet service providers are not generally considered operators because they usually subcontract the service to another party.
(2) ATU-R (downstream receiver) and ATU-C (upstream receiver) are limited to 14.5 dB maximum gain reduction request, which often is insufficient to implement the intent of MAXSNRM. Furthermore some ATU-R modems ignore MAXSNRM and there has never been an interoperability test to declare such modems non-compliant with G.992.1 that actually verifies the margin does not exceed MAXSNRM.
(3) Downstream ATU-C transmitters reduce power by up to 12 dB according to algorithms in the annexes of G.992.1 (the most popular of which are Annexes A, B, and C) if the early-training received upstream signals are large, indicating that the loop is short. The algorithms of Annexes A, B, and C are blind to downstream channel-output noise, and so are very “timid” in reducing power and almost always do not reduce power nearly enough—for instance, the widely used Annex A algorithm only applies to lines less than 3000 feet in length and thus fails to accommodate many situations in which longer lines might need power reduction as well.
(4) An initial, flat PSD upper bound or “mask” can be programmed according to a MAXNOMPSD parameter, which is between −40 and −52 dBm/Hz in 2 dB steps. The MAXNOMPSD value is set by the operator and is the maximum value that NOMPSD can assume at the initiation of transceiver training. The modem manufacturer can set a modem to use a lower NOMPSD value, in which case NOMPSD<MAXNOMPSD. The MIB/telcom operator can only set MAXNOMPSD in earlier systems, but cannot force the NOMPSD value itself. In ADSL1, NOMPSD is communicated during training to the receiver (for subsequent attenuation calculation) by the transmitter. Again, there is no MIB parameter that directly specifies NOMPSD in ADSL1. Usually, MAXNOMPSD is the NOMPSD that is used in the very first transceiver training phase of ADSL1, but the NOMPSD level (only restricted to be less than or equal to MAXNOMPSD) is decided by the modem manufacturer in design and not the operator. Thus, by setting MAXNOMPSD the operator is assured that NOMPSD will be reduced to an upper limit level of NOMPSD−2nPCB dBm/Hz, as defined, with nPCB=0 to 6 (that is, if NOMPSD is −40, then PSD power can be dropped by 0, 2, 4, 6, 8, 10 or 12 dB).
(5) Some manufacturers' ATU-R receivers ignore the MAXSNRM altogether and never request the 14.5 dB power reduction despite such power reduction being mandated by the operator and by standards.
(6) Gain swapping during live operation may be limited and recommended but not required in Annex A) to allow only +/−2.5 dB gain adjustments after training in SHOWTIME. Thus, gain swapping is limited in some modems to a total of +/−2.5 dB in ADSL1. If gain is not reduced enough in training for any reason (for example, a spurious noise was present), then further lowering will not occur unless the modem retrains. A record of retrains (a retrain count) may be maintained by the DSL system as an indication of how many retrains are performed in a given period of time and as an indication that the MAXSNRM level might be set too low if the retrain count is high. Gain-swaps are applied successively in ADSL1 (so that they build upon one another), but the total gain reduction of any SHOWTIME succession of gain swaps is often limited to a maximum of +/−2.5 dB with respect to training. Some vendors, however, do request a succession of gain reductions that lead to the full ADSL 1-allowed gain reduction of −14.5 dB during SHOWTIME. When asking for lower than −2.5 dB changes, the receivers in such full-range gain-swap systems need to be smart enough to adjust internal signal processing to prevent intersymbol interference from the fixed synch symbol (that is never reduced in power during SHOWTIME by gain swaps, unlike the other 68 live-data symbols). Some deficient receivers ask for the power reduction, but then fail to adjust themselves internally when a lower than −2.5 dB gain reduction is implemented (and operators do not easily know on which lines these deficient receivers are located). Such a deficient ATU-R then drops the DSL connection because it assumes the line went bad, when, instead, the problem is due to the ATU-R's faulty implementation of asking for a gain reduction it cannot handle. Due to this problem, service providers may force DSLAM providers to always ignore power reduction requests that exceed the +/−2.5 dB range during live operation (and do so network wide because they do not know where the deficient receivers are located, thus limiting also all good receivers). This further limits power reduction if the service provider selects this option to not have to change the deficient receivers already deployed in their network.
ADSL2-G.992.3 Standard (Also Referred to Herein as the “ADSL2 Standard” or “ADSL2”):
(1) Has a MAXSNRM setting, but this function still is left to a DSL receiver to implement.
(2) The ATU-R (as the downstream receiver) and ATU-C (as the upstream receiver) are limited to a 14.5 dB maximum power reduction request for gain settings, which are now set absolutely in gain swaps and not done relative to last gain swap. The range is now from −14.5 to [+2.5+EXTGI], so it still is limited to a maximum power reduction of 14.5 dB (EXTGI≧0, and usually is equal to 0; EXTGI is something the transmitter tells the receiver during early training it can accommodate during later gain swapping). A larger EXTGI of up to the limit of 18 dB allows a modem that has reduced power for any reason to increase its power during live operation to respond to a new larger noise that may have occurred during live operation.
(3) Power Cut Back (PCB) in ADSL2 allows the receiver to reduce power (only during training) by an additional 0, 1, . . . , 40 dB, so the ability to observe MAXSNRM is improved. The ADSL2 standard provides that the largest PCB requested by either transmitter or receiver should then be implemented. MAXNOMPSD is still only one operator-controlled parameter in ADSL2 that applies to the entire band, but a wider range of this parameter is accommodated in ADSL2 than in ADSL1.
(4) The initial flat PSD mask can be programmed according to a MAXNOMPSD parameter, which is between −40 (and −37 in certain reach-extended Annexes of ADSL2 known as READSL) and −60 dBm/Hz in 0.1 dB steps.
(5) Some manufacturers' ATU-R receivers may still ignore power back off and it is unfortunately not tested, even in the DSL Forum's new test procedure called WT-85 (though there is a test, nearly all would pass it and there is no verification in that test that the MAXSNRM is observed). No band preference (that is frequency-dependent imposition of a PSDMASK) is possible in the ADSL2 standard itself.
(6) Gain swapping during live SHOWTIME is no longer limited to +/−2.5 dB and all symbols (there is still a synch symbol every 69th) have the same level. However, only gain-swapping of up to a reduction −14.5 dB relative to training levels (not last level as in ADSL1) is possible. A gain increase of up to 2.5+EXTGI is particularly useful if the modem started at very low power and a noise arose. If EXTGI is large, then the modem can recover without retraining. EXTGI is limited to 18.0 dB in ADSL2.
ADSL2+-G.992.5 Standard (Also Referred to Herein as the “ADSL2+ Standard” or “ADSL2+”):
(1) Same as ADSL2, except for the introduction of the PSDMASK parameter that is implemented through the tssi parameters. The tssi are additional parameters like the gains in gain swapping, except that the tssi can be fixed externally.
VDSL1, VDSL2, HDSL and SHDSL
The current version of the proposed VDSL1 standard, or G.993.1 has limited definition of MIB-controlled (or operator-controlled) procedures for power reduction (the DSLAM or line terminal (LT) modem manufacturer has a complete PSDMASK specification internally but access to it via MIB is at best not-yet-well-defined in G.993.1). An enumeration of the G.993.1 standard's maintenance capabilities appears in DSL Forum Document TR-057, however the MIB controlled section of TR-057 is currently empty. Thus, VDSL has no standardized mechanism for setting MAXNOMPSD externally, but does have an internal mechanism for reducing power in 0.25 dB steps (called manual power control) between 0 and 40 dB for upstream and 0 and 12 dB for downstream, with respect to nominally imposed standard limits (there are two mask levels and corresponding transmit power levels downstream and upstream that can be set programmably in G.993.1 compliant modems—so the power reduction is with respect to these, some of which are not yet specified). VDSL also specifies a MAXSNRM (but again it is not clear who specifies this). Thus, VDSL has many of the same capabilities as ADSL1 and ADSL2/2+. These capabilities may become standardized for operator interface in an MIB in future documents that could allow many of the same capabilities as ADSL1, ADSL2 and ADSL2+. However, VDSL1 also does not have the rich set of reported diagnostics of ADSL2 and ADSL2+, or apparently even ADSL1, so the ability to diagnose a problem accurately may be more difficult. Again, future generations of TR057 or G.997.x may address these deficiencies of current VDSL MIB interfaces.
VDSL2 is still in very early stages, but it appears that it will have essentially the same MIB features as ADSL2+. HDSL does not appear in any form to have any power back off features. HDSL (now upgraded to SHDSL, G.991.2) has a target SNR (or TSNRM) and a reported SNRM, but no MAXSNRM. The bandwidth is fixed for any of a few data rates (basically 384, 768, 1.5, 3, . . . ) symmetric and the same modulation in both directions with some standardized shaping. A flat PCB of 0, . . . , 31 dB can be imposed. There is no FEC to protect against impulses at all, so it is unlikely that PCB is used much. Moreover, SHDSL tends to run at the maximum rate possible on short lines, so its margin typically will be close to the TSNRM of 6 dB.
DSM Report, still in its draft stages, currently has all the MIB capabilities of ADSL2+ in it in both directions, and applies to all DMT transmission methods, ADSL1 through VDSL2 and beyond. FEC also can be specified.
As will be appreciated by those skilled in the art, in many DSL systems, including ADSL1 and ADSL2 systems, operational characteristics and rules typically have been set for a static mode of operation to accommodate worst-case scenarios in the systems. That is, users do not always realize the full benefits of DSL systems because of inadequate standards, equipment limitations and the deficiencies of generally accepted operational procedures and conventions. For example, power-margin limits are rarely observed or may be in conflict with or between various standards or interpretations of those standards. Such disregard for service-provider-imposed and/or standards-decreed limits creates problems for users, including excessive crosstalk. Similarly, impulse noise can be a significant problem in some DSL systems. To address impulse noise, current systems use manufacturer-supplied default settings for many operating parameters (such as margin). The applicable standards intended to allow the service provider to set these parameters, yet they may or may not be implemented properly by the various vendors' DSL modems or equipment.
Even where a majority of users (that is, their modems) in a binder are standards compliant, a single user can prove to be a significant source of service deterioration or other damage to other users' DSL service. For this reason, while the standards have provided guidance, even minimal non-compliance can present significant problems in current systems.
Static operation (for example, when DSL service uses manufacturer-set default settings in a DSL modem) means that the DSL service cannot adjust and/or adapt to changing line and environmental conditions in the subscriber line, again forgoing and/or diminishing the benefits available in such DSL systems and failing to realize the potential available to one or more users in such systems. As will be appreciated by those skilled in the art, the widely varying standards, equipment, implementation rules (or lack thereof) and practices mean that, despite detailed standards concerning operation of these various DSL systems, consistent service and service quality is challenging. Because the modems and other equipment may or may not actually comply with the appropriate standards and, as or more importantly, the fact that a user's neighboring lines may or may not use standards-compliant equipment and practices, many users suffer poor or less than optimal service.
Systems, devices, methods and techniques that allow users to adjust and adapt transmission power margin(s), power spectral densities, and the like dynamically to changing DSL environmental and operational situations would represent a significant advancement in the field of DSL operation. Moreover, monitoring and evaluation of the power, margins, etc. used in the DSL environment and operation by an independent entity can assist, guide and (in some cases) control users' activities and equipment, and likewise would represent a significant advancement in the field of DSL operation.